In continuously variable power-branched transmissions or building machine transmissions known from practice, hydraulically actuated frictional shifting elements are commonly used, which in the area of a piston chamber are filled with hydraulic fluid and acted upon by appropriate actuation pressure. The extent of filling of a piston chamber of such a shifting element significantly influences the transmission capacity of the shifting element and the torque that can be transmitted by the shifting element concerned at the time. In addition, the actuation sequence of a shifting element affects its closing behavior and thus also the shifting comfort, which is determined decisively by the torque applied in each case in the area of a drive output of a vehicle, or the variation thereof.
Since, as is known, manufacturing tolerances of mass-produced components used in shifting elements and hydraulic lines can sometimes show an undesirable degree of scatter, for each shifting element of a transmission an individual calibration should be carried out in order to know the particular filling behavior of the shifting elements and to be able to actuate the shifting element so as to achieve an appropriate level of shifting comfort. The parameters that characterize the clutch filling in each case and which are determined by the calibration are stored in the area of a non-volatile memory of a transmission control system and taken into account when carrying out the actuation of a shifting element during every gearshift.
In this context, two parameters in particular are characteristic for the filling behavior of a shifting element. The first parameter is the so-termed rapid filling time, during which a shifting element is acted upon by a so-termed rapid filling pulse in order to fill the shifting element within short operating times. The piston chamber of a shifting element is acted upon by a defined rapid filling pressure for the rapid filling time. The rapid filling phase is followed by a so-termed filling equalization phase, during which the actuation pressure applied in the area of the piston chamber falls from the level of the rapid filling pressure to a level of a filling equalization pressure that represents the second parameter, and remains there for a further, defined filling equalization time. At the end of the filling equalization phase, ideally the shifting element is in a defined operating condition in which the transmission capacity of the shifting element is essentially equal to zero, and starting from which, an increase of the actuation force of the shifting element results in an immediate increase of the transmission capacity of the shifting element.
In calibration methods known until now the rapid filling time is determined iteratively. For this, starting from a very short rapid filling time the rapid filling time is increased step by step until at the end of the rapid filling phase the shifting element transmits a torque. This operating condition of a shifting element can be verified in a simple manner as a function of a variety of rotational speed variations of components of the transmission. During this the rotational speed is monitored by a suitable recognition function. In each case the calibration process is based on the assumption that the shifting element whose rapid filling time is currently being calibrated changes to a completely drained operating condition between the individual iteration steps for determining the rapid filling times.
However, besides the filling behavior the draining behavior as well has a decisive influence on the quality of gearshifts. This results from the fact that particularly in the case of building machines one and the same clutch is engaged rapidly in succession in the force flow of a vehicle drive-train, which is the case particularly when several reversing processes are carried out one after another. If one and the same clutch is engaged and disengaged in each case after the passage of very short operating times, it is possible that the clutch will not drain completely before being re-engaged, and therefore has to be engaged again from a so-termed partially filled operating condition. This, however, has the result that previously known engagement routines for respective shifting elements to be engaged are not suitable for preparing a shifting element to the extent required for a desired high comfort level of the engagement.